Pressure measuring instruments often are installed in places where they are subject to widely varying environmental conditions, such as changing ambient temperatures. Consequently, it is not uncommon for the instrument zero-set and span calibrations to drift or in some way be offset over time, resulting in erroneous readings or measurements in which system operators lack full confidence. Since the instruments frequently are in locations which are not readily accessible to routine maintenance, zero-set and calibration errors in many cases have not been easily correctable by operating personnel. Moreover, calibrating the span of instruments of the kinds available heretofore typically has involved relatively complex, and time-consuming procedures which may interrupt an on-line operation. For these reasons, the confidence level an operator has in the measurements may be less than is desirable.
Because of the importance of minimizing measuring errors, various proposals have been made for solving or ameliorating these problems. For example, remotely-operated zero-set apparatus now is available for use with differential-pressure transmitters. Such apparatus comprises a remotely-controllable pressure manifold which, upon demand, blocks the low-pressure process line and bypasses the high-pressure process to the high and low sides of the transmitter, producing a zero-differential-pressure condition. If under such circumstances the transmitter output signal differs from the indicating zero differential pressure, the measured error value may be stored (as in a computer) and thereafter used to correct the output signal when measurements are resumed.
However, such remote-setting of instrument zero does not correct for the errors in span calibration. Nor does such a correction necessarily enhance operator confidence in measurement accuracy. Thus, in an effort to avoid the defects of such errors, differential pressure-transmitters have been designed to include one or more common-sensing elements (such as temperature and static pressure sensors) arranged to function with associated devices to automatically adjust the transmitter output signal in response to changes in the sensor's ambient conditions. For example, the transmitter output signal may be controllably altered in accordance with a predictive algorithm stored in a microprocessor forming part of such instrument or system.
Although such compensator arrangements have improved the accuracy of the pressure measurement they have not eliminated the problem. In part this is because such techniques are not capable of achieving the desired accuracy, particularly since there remain other uncompensated variables. Thus, the need for instrument recalibration from time to time is not eliminated. Moreover such compensating arrangements are relatively costly to implement. U.S. Pat. Nos. 4,638,656 and 4,604,891, to Sgourakes et al and assigned to the assignee of the present invention, provide a pressure measuring instrument with a control room signal-activatable device for developing a highly accurate and repeatable reference pressure to be applied to the field-based pressure-sensing elements of the instruments. The reference pressure device of these patents comprises a vertically-oriented cylindrical tube containing a fill fluid which communicates with the fill fluid in the instrument. A ferromagnetic ball is raised in the cylinder by an applied electromagnetic field and then released to produce the reference pressure.
The '891 patent discloses a reference-pressure device of the type basically similar to the device disclosed in the '656 patent. However, the '891 patent includes an improvement wherein the magnitude of the reference pressure pulse is substantially increased by suspending an ancillary weight from the ball so as to augment the force supplied to the falling ball. The increase in force, in effect, increases the apparent density of the ball. Since the magnitude of the pressure pulse is proportional to the relative densities of the ball and the fill-liquid, the increase in apparent density correspondingly increases the reference pressure pulse generated.
These known prior art calibration techniques require one or more remotely operated valves and a process manifold.